The manufacture of a ceramic matrix composite (CMC) part typically includes laying up pre-impregnated composite fibers having a matrix material already present (prepreg) to form the geometry of the part (pre-form), autoclaving and burning out the pre-form, infiltrating the burned-out pre-form with the melting matrix material, and any machining or further treatments of the pre-form. Infiltrating the pre-form may include depositing the ceramic matrix out of a gas mixture, pyrolyzing a pre-ceramic polymer, chemically reacting elements, sintering, generally in the temperature range of 925 to 1650° C. (1700 to 3000° F.), or electrophoretically depositing a ceramic powder. With respect to turbine airfoils, the CMC may be located over a metal spar to form only the outer surface of the airfoil.
Examples of CMC materials include, but are not limited to, carbon-fiber-reinforced carbon (C/C), carbon-fiber-reinforced silicon carbide (C/SiC), silicon-carbide-fiber-reinforced silicon carbide (SiC/SiC), alumina-fiber-reinforced alumina (Al2O3/Al2O3), or combinations thereof. The CMC may have increased elongation, fracture toughness, thermal shock, dynamic load capability, and anisotropic properties as compared to a monolithic ceramic structure.
Conventional mid-span, damper-style turbine blades typically do not require significant shank height due to damping criteria, and the shank length is conventionally targeted to be around 10% of the overall blade length, regardless of the material out of which the blade is made.
Decreasing the shank length on a CMC blade may have two negative side effects, namely increasing the local interlaminar tension (ILT) and causing drastic ply drop regions, which are typically prone to defects. A longer shank improves the ply drop transition at the leading edge (LE) and the trailing edge (TE) of the hub of the CMC blade, but lengthening the dovetail shank results in greater material usage and may be prohibitive in the case of CMC blades.